CMOS image sensors as disclosed in, e.g., JP 2000-244818 and JP 2000-175107 are known as semiconductor devices. A pixel cell of these CMOS image sensors includes a light-receiving photo diode, and an amplification circuit that amplifies electric charge generated by the photo diode in accordance with the light amount, and transmits the amplified electric charge to an A/D converter.
A CMOS image sensor like this has the problem that when the pixel cells are downsized, noise generated from circuit elements deteriorates the quality of a sensed image. When the pixel cells are downsized, the photo diodes are also downsized. This decreases the light amount per unit time that can be received by the image sensor. On the other hand, a MOS transistor forming the pixel cell generates noise to some extent. As the pixel cells are downsized, however, the ratio of an optical signal to the noise of the MOS transistor, i.e., the S/N ratio significantly lowers. Noise that particularly exerts influence on the image quality of an image sensor is called 1/f noise because the frequency component is proportional to the reciprocal of the frequency. The origin of the 1/f noise is presumably charge and discharge that occur at a certain probability with respect to the interface state existing in the interface between a gate insulating film and semiconductor in a MOS transistor. To decrease this interface state density, the manufacturing steps of the MOS transistor are improved and controlled. However, it is difficult to make the interface state density lower than certain amount of density. Accordingly, the S/N ratio significantly decreases as the pixel cells are downsized as described above.
Since the origin of the 1/f noise is the interface state in the interface between the insulating film and semiconductor, the generation amount of the 1/f noise changes in accordance with the structure of the MOS transistor forming the pixel cell. The structures of MOS transistors are classified into an enhancement structure and depletion structure. In an enhancement transistor conventionally used in an image sensor, the probability of charge and discharge of the interface state increases, so the 1/f noise increases more easily than in a depletion transistor.
Furthermore, since the conventional pixel cell uses the enhancement MOS transistor, the operating point of an amplification circuit must be set at a high voltage. However, peripheral circuits such as an A/D converter operate at a lower voltage owing to LSI scaling down. Accordingly, the high-voltage operation of the amplification circuit hardly matches the low-voltage operation of the peripheral circuits in the scaled image sensors.